Polyamide 4 has a feature of being synthesized from biomass. Specifically, a monomer, 2-pyrrolidone, used as a raw material therefor can be obtained from γ-aminobutyric acid that is made by decarboxylating glutamic acid, which is industrially manufactured by fermenting biomass (i.e., glucose). Polyamide 4 has excellent thermal and mechanical properties because of the strong intermolecular hydrogen bonds due to its macromolecular chain structure comprising short methylene chains. Further, among polyamides, only polyamide 4 is biodegraded by microorganisms in the natural environment, e.g., activated sludge, seawater, and soil. The polymerization mechanism involves bonding of an initiator to form propagation species, and the macromolecular design of polyamide 4 is therefore easy.
Polyamide 4 was synthesized for the first time in 1956 by William O. Ney et al. by using metallic potassium as a basic catalyst and an acyl group-containing compound as an activator, to thereby cause ring-opening polymerization of 2-pyrrolidone to proceed by an activated monomer mechanism (PTL 1). Based on this technique, new technologies pertaining to new catalytic systems, polymerization procedures, copolymerization with ε-caprolactam and the like were continually developed from the 1950s through the 1990s with the aim of increasing the molecular weight, controlling polydispersity, and simplifying the manufacturing process (NPL 1 to 5). Generally, such technologies were intended to manufacture linear polyamide 4 as a commodity material to fabricate fibers and films by melt processing, which is economically advantageous. Although some of the research led to a technology development whereby melt spinning of polyamide 4 became possible, its commercialization was hampered by drawbacks, such as insufficient strength and difficulty in molding.
For overcoming these drawbacks, PTL 2 teaches that polymerization of 2-pyrrolidone by using a basic polymerization catalyst and a carboxylic acid compound produces a uniquely structured 2-pyrrolidone polymer containing structures derived from the carboxylic acid compound, and that the physical properties, such as thermal stability and moldability, of the thus-obtained 2-pyrrolidone polymer can be controlled and improved.
Taking advantage of the ease of macromolecular design, the present inventors used a polyfunctional initiator (1,3,5-benzene tricarbonyl trichloride etc.) as disclosed in PTL 2 to develop polyamide 4 having a 3-branched structure. The development of 3-branched polyamide 4 led to success in achieving a tensile strength higher than that of linear polyamide 4 of an equivalent molecular weight (approx. Mw 100×103) due to the tangled molecular chains made by introducing the branched structure. Moreover, specimens of 3-branched polyamide 4 were made by injection molding for the purpose of evaluating the basic physical properties of the specimens, such as heat resistance and strength. As a result, the specimens of 3-branched polyamide 4 exhibited performance equivalent to or better than that of polyamide 6, which is a typical engineering plastic.
PTL 3 reports that when carrying out polymerization of 2-pyrrolidone, copolymerization with ε-caprolactam was performed using a basic polymerization catalyst and an initiator that has a structure with two or more branches to control the macromolecular chain structure and the macromolecular chain composition, thereby enabling modification of the physical properties (i.e., mechanical properties and thermal properties).